1. Field of the Invention
This invention relates to a system and method for exchanging data over a communication channel and more specifically to modems equipped to exchange secondary channel information in an efficient manner.
2. Description of the Prior Art
It should be noted at the outset that the primary channel messages referred to herein are the synchronous data which is being communicated over the channel, while the secondary channel messages referred to the other additional asynchronous information being exchanged between the transmitter and receiver modem circuits such as "a retraining message is coming".
Primary channels such as those used in the present invention transmit data information at a constant synchronous rate. Thus, since it is known when each bit is to be communicated, the filters and detectors can be matched to the bit rate. The synchronous system requires, of course, that timing information be provided to the filters and detectors of the modem.
On the other hand, with asynchronous transmission in a secondary channel message, the timing of individual bit transmissions is not accurately known and consequently prior art systems have framed transmission bytes of, e.g., eight bits with added start and stop bits. However, in the prior art previous example of an eight bit byte, the start and stop bits render the message transmission 20% less efficient.
The typical prior art system shown in FIG. 5 for communicating the aforementioned primary data and secondary message channels over a communications link comprises a diagnostic microcomputer (DMC) 100 and a UART 101 secondary channel data in an asynchronous 10-bit signal which generates eight character bits and stop and start bits. The transmitter circuit conveys this information in the form of a serial bit stream at a bit rate of, for example, 110 bits/second with no associated clock signal. In a universal asynchronous receiver transmitter chip (UART) 101 the asynchronous serial data at 110 bits/second is then conveyed to an LSI module secondary channel frequency shift key transmitter 102 arbitrarily designated "secondary channel" which samples the bits at a high rate (for example, 38.4 KHz) and frequency-shift keys the incoming bits into two carrier frequencies (for example, 450 Hz or 390 Hz) depending on whether they are "1" or "0" bits for transmission over the channel. The transmitter circuit also contains a primary modem transmitter 103 for quadrature amplitude modulation which accepts generated data for conventional synchronous transmission, with timing information, of the generated data over the channel. Adder 104 adds data from transmitter 102 and primary modem 103 and transmits the results to the receiver circuit 105.
The receiver circuit 105 for the prior art system comprises a primary modem receiver 106 and filters 107 and proxy frequency shift key receiver 108 which accept the synchronous signal from the aforementioned primary modem transmitter and decodes and demodulates the signal in a manner well known to those skilled in the art. Primary data is received at primary modem 109. The secondary asynchronous signal, which has been frequency-shift keyed at the transmitter, is demodulated in a proxy frequency shift key receiver 108 to derive asynchronous bits which are sampled at, for example, about 1600 Hz. The demodulated signal is then conveyed as a series of bits at for example, 110 bits/second, to a receiver universal asynchronous receiver transmitter module 111 wherein a start bit for a character is determined by correlating the bit pattern over many bit times, and then eight bits after the start bit are sampled. Finally, the eight bit character information is conveyed to a receiver diagnostic microcomputer 112.
In this prior art system the primary data and secondary data are communicated independently on the communication channel by means of band-separation filtering. As it is necessary in this prior art system to create an independent secondary channel which occupies additional bandwidth, a wider total bandwidth is required in order to maintain performance. Additionally, filters are required for both the primary and secondary channel as shown n FIG. 5. Furthermore, the secondary channel is always transmitting, regardless of whether there is data to be transmitted or not, thereby requiring the increased bandwidth at all times.